Rockwool question

I've never read this in anything but a friend mentioned it so now I want to get someone else's opinion. I have rockwool in an ebb & flow table for my clones, but about half of the rockwool cubes are above rock, can this cause any problems/mildews/fungus etc? I would prefer to save the 30 bucks on buying more rocks as im strapped for cash, but if necessary I will make it happen.

Thanks in advance~